Methods for measurement, analysis and assessment of ground structure

ABSTRACT

For determination of the laminar structure and other characteristics of a ground, oscillation sensors for detecting the vertical component or vertical and horizontal components of oscillations are disposed at three or more points within a comparatively small area at ground level to simultaneously measure microseisms at the respective points, if necessary with locations of said points being varied and measurements be performed at the respective locations and the vertical oscillation data or vertical and horizontal oscillation data thus generated are analyzed. This method not only permits an expedient, positive and accurate assessment of the laminar structure and other characteristics of the ground but also permits the measurement of ground structure even when the area available for measurement is small.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to methods for measurement,analysis and assessment of ground structure.

[0003] 2. Background Art

[0004] To determine the structure and characteristics of the surfacelayer of the ground, it has heretofore been necessary to drill aplurality of holes in the ground and insert or indwell instruments andancillary devices into the holes and also to employ a source ofoscillation for generating waves to be measured. It should, therefore,be obviously of great benefit if it were ever possible to knowthe,structure and characteristics of the surface layer of the groundwithout any of such efforts and contrivances. To this end, it might becontemplated to measure and analyze the oscillatory waves or microseismsobserved in the surface layer of the ground. However, up to the presenttime, there is not known a measuring technology or a system ofinstrumental analysis, by means of which short-period microseisms couldbe measured with sufficient accuracy to estimate the tectonic and othercharacteristics of the surface layer of the ground inclusive of thevelocity of shear (S) waves.

[0005] Furthermore, the prior art system designed to simultaneouslymeasure short-period microseisms at a plurality of points to therebydelineate the ground structure has the following disadvantages.

[0006] A. Since the spacings of oscillation sensors must be large, thatis to say the scale of measurement must be large, a large free area isrequired and it is time-consuming to connect the cords from therespective oscillation sensors for cental analysis. Moreover, becausethe large number of oscillation sensors are required, the analyticalprocedure is scaled up to the extent that the method cannot be utilizedadvantageously in the field.

[0007] B. Since the network of oscillation sensors is not tailored toimprovement in measuring accuracy or resolution of high-order modes, thereliability of the measured dispersion curve is low.

[0008] C. Short-period microseisms include not only substantive wavessuch as dilatational (P-) waves and S-waves but surface waves such asRayleigh and Love waves. Since the confirmation or judgement of whetherthe detected waves are surface waves or not is not performed formed atmeasurement or analysis, it is impossible to judge whether thedispersion curve obtained is relevant to the surface waves and, hence,to accurately determine the ground structure.

[0009] D. Since short-period waves include higher-modes surface waves,it is impossible to compute an accurate surface wave dispersion curveand, hence, to ascertain the surface layer structure of the groundunless some means for removing or separating these waves are available.

[0010] E. Since the dispersion curve cannot be computed real-time fromfield values, there is no knowing whether measurements were made withacceptable accuracy.

OBJECT OF THE INVENTION

[0011] It is an object of the present invention to provide a method formeasurement, analysis and assessment of ground structure whereby thelaminar structure and characteristics of the ground can be determinedquickly, expediently and accurately through simultaneous ground-levelobservation and analysis of short-period microseisms even in a confinedavailable land.

[0012] It is another object of the present invention to provide a methodfor measurement, analysis and assessment of ground structure which doesnot call for drilling of holes or the use of an oscillation source.

[0013] It is a further object of the invention to provide a method formeasurement, analysis and assessment of ground structure in which theaccuracy of measurement and analysis has been improved by using anetwork of substantially equi-spaced oscillation sensors whichfacilitates field sensor positioning as well.

[0014] It is another object of the present invention to provide a methodfor measurement, analysis and assessment of ground structure by whichthe laminar structure and characteristics of the ground can beascertained with accuracy through selective extraction of surface waves.

[0015] It is still another object of the present invention to provide amethod for measurement, analysis and assessment of ground structure bywhich the accuracy of assessment of the ground structure can be improvedby a collective analysis of data obtained by repeated measurement usingdissimilar sensor-to-sensor distances.

[0016] It is still another object of the present invention to provide amethod for measurement, analysis and assessment of ground structure bywhich the laminar structure and characteristics of the ground can beevaluated conveniently and accurately on a real-time basis.

SUMMARY OF THE INVENTION

[0017] The present invention is predicated on the facts that whensurface waves such as Rayleigh waves are propagated through a horizontalmulti-layer ground structure, phase velocity (the velocity ofpropagation) varies with different wavelengths (dispersions), that theapparent phase velocity can be computed by averaging the correlationcoefficients of the waveforms measured by equi-spaced oscillationsensors in different directions, that this apparent phase velocityvaries with different sensor-to-sensor differences due to the influenceof higher-order modes, and that when the horizontal and verticalcomponents of the motion of a particle on the ground surface due topropagation of oscillations are detected and synthesized, the locus ofrotation or orbit of a particle on the ground surface can beconstructed, and that such surface waves are present in short-periodmicroseisms as well. As such, the present invention is directed to amethod for measurement, analysis and assessment of surface wavedispersion curves which serve as bases for evaluation of the tetonic andelastic characteristics of the ground by repeated simultaneousmeasurement of short-period microseisms at a plurality of points in asimple small-scale measuring system wherein 3 or more oscillationsensors are disposed at substantially equal sensor-to-sensor distanceswithin a comparatively small area and inverse analysis of the surfacewave dispersion curves at the points of measurement to compute theS-wave velocity of the ground and evaluate the laminar structure andfeatures of the ground.

[0018] In the first aspect, the present invention provides a method forestimating the laminar structure and characteristics of a ground whichcomprises disposing an oscillation sensor adapted to detect the verticalcomponent or the vertical and horizontal components of oscillations ateach of at least 3 points within a comparatively narrow area and therebymeasuring microseisms at such plural points simultaneously, if necessarychanging the points of measurement and repeating the measurement, andanalyzing the vertical oscillation data or vertical and horizontaloscillation data.

[0019] In the second aspect, the present invention is directed to amethod for estimating the laminar structure and characteristics of aground according to said first aspect wherein said microseisms to bemeasured are short-period microseisms observed at ground level.

[0020] In the third aspect, the present invention is directed to amethod according to said first aspect wherein two oscillation sensorsare disposed on each of 2 or more straight lines passing through theground to be measured and dividing the azimuth of 360 degrees into aneven-number of sections or straight lines paralleled thereto in such amanner that all sensor-to-sensor-distances are equal on each of saidstraight lines.

[0021] In the fourth aspect, the present invention provides a methodaccording to said first aspect wherein said oscillation sensors aredisposed at 3 or more points with equal spacings on the circumference ofa circle and at one point in the center of the circle, or at a total ofat least 4 points.

[0022] In the fifth aspect, the present invention provides a methodaccording to said first aspect wherein said oscillation sensors aredisposed at a pices of a regular triangle and the center of gravitythereof or a total of 4 points.

[0023] In the sixth aspect, the present invention is directed to amethod for estimating the laminar structure and characteristics of aground which comprises analyzing vertical oscillation data or verticaland horizontal oscillation data as measured simultaneously withoscillation sensors, judging whether the oscillation waves detected bysaid oscillation sensors are surface waves or not, and extracting andanalyzing the surface waves.

[0024] In the seventh aspect, the present invention is directed to amethod for evaluating the laminar structure and characteristics of aground which comprises analyzing the correlation coefficients and phasedifferences of oscillation data measured by equi-spaced oscillationsensors at respective frequencies, performing the analysis for each ofvarying sensor-to-sensor distances, analyzing and computing thefundamental mode of surface waves based on results of the respectiveanalyses and, if necessary, analyzing and completing the higher-modewavelengths.

[0025] In the eighth aspect, the present invention relates to a methodfor estimating the laminar structure and characteristics of a groundwhich comprises performing the measurement and analysis of microseismsrepeatedly until it can be concluded that a relation between phasevelocity and wavelength such as to give a sufficiently accurateevaluation of ground structure can be achieved.

[0026] Thus, in accordance with the present invention, oscillationsensors for detecting the vertical component or the vertical andhorizontal components of a vibration are disposed at 3 or more pointswithin a comparatively small area at ground level and the microseismsare measured simultaneously at said points, if necessary changing thelocations of said points of measurement and the resulting verticaloscillation data or vertical and horizontal oscillation data areanalyzed. By measuring microseisms with a small network of oscillationsensors, the accuracy of assessment of the surface structure of theground can be improved and a system enabling a rapid field measurement,analysis and assessment can be constructed. Therefore, the laminarstructure and characteristics of the ground can be estimatedexpediently, positively and accurately.

[0027] Furthermore, the present invention is concerned with short-periodmicroseisms which can be easily measured at ground level and as suchdoes not require drilling and placement of instruments in the drilledholes or the use of an oscillation source so that the measurement andanalysis of ground structure can be easily performed in any location.

[0028] Moreover, since the network of oscillation sensors formeasurement can be set up in a comparatively confined area, themeasurement is facilitated and can be performed even in a small plot ofland.

[0029] In addition, when the vertical and horizontal oscillation dataare analyzed and the locus of rotation of a particle on the groundsurface due to propagation of oscillations is constructed and theanalysis and assessment are carried out on confirming that the measuredwaves are surface waves, the P and S waves can be differentiated and thesurface waves only be positively measured to provide accurateinformation on the ground structure.

[0030] Moreover, when oscillation sensors are disposed in a networkwherein sensor-to-sensor distances are substantially equal and themeasurement of waves is repeated varying the sensor-to-sensor distances,the high-mode surface waves can be isolated from the fundamental mode,with the result that a highly accurate relation of dispersion wave towavelength can be obtained. It is because the analytical system isconstructed using a microcomputer that the assessment can be obtained onthe spot.

[0031] Moreover, when it is so arranged that the measurement ofmicroseisms, amplification of detected signals and the analysis andassessment of data are automatically performed as a sequence, therelationship between dispersion curve and wavelength can be moreaccurately determined in the field.

EFFECTS OF THE INVENTION

[0032] In accordance with the present invention, a plurality ofoscillation sensors for detecting the vertical component or vertical andhorizontal components of oscillations are arranged at 3 or more pointsin a comparatively small area, the measurements of microseisms at therespective points are simultaneously performed, if necessary varying thelocations of said points and repeating the measurements, and themeasured vertical oscillation data or vertical and horizontaloscillation data are analyzed to ascertain the laminar structure andcharacteristics of the ground. Therefore, the laminar structure andother characteristics of the ground can be determined not onlyexpediently, positively and accurately but even when the area availablefor measurement is confined.

[0033] In the second aspect of the present invention wherein saidmicroseisms to be measured are short-period microseisms observed on thesurface of the ground, the laminar structure and other characteristicsof the ground can be determined without calling for drilling of holes orthe use of an oscillation source in addition to realization of theeffects achieved by the first aspect of the invention.

[0034] According to the third aspect of the invention, two oscillationsensors are arranged on two or more straight lines passing through theground to be measured and dividing the azimuth of 360 degrees into aneven-number sections or straight lines parallel thereto in such a mannerthat the distance between the oscillation sensors is constant on each ofsaid straight lines, with the result that, in addition to the effects ofthe first aspect of the invention, the field installation of theoscillation sensors is facilitated and, moreover, because thesensor-to-sensor distance is constant in different directions, theaccuracy of measurement and analysis is improved.

[0035] Comparable effects are obtained in the fourth and fifthinventions.

[0036] In accordance with the sixth aspect of the present invention, thevertical oscillaton data or vertical and horizontal oscillation datameasured simultaneously by said oscillation sensors are analyzed to seewhether the oscillation waves detected by the oscillation sensors aresurface waves or not and the laminar structure and other characteristicsof the ground are still more accurately estimated from the accuratedispersion curves constructed by extraction and selective analysis ofthe surface waves.

[0037] In accordance with the seventh aspect of the present invention,the correlation coefficients and phase differences of the oscillationdata measured by oscillation sensors disposed at equal spacings for therespective wavelength are analyzed, varying the sensor-to-sensordifference and repeating the measurements and, based on results of therespective analyses, the fundamental mode of the surface waves isanalyzed and computed. If necessary the wavelength of the higher mode isanalyzed and computed to assess the laminar structure andcharacteristics of the ground. Therefore, in addition to the effects ofthe first to the sixth aspect of the invention, the higher mode surfacewaves which could not be easily isolated in the past can be neatlyisolated for insuring a further improvement in the accuracy ofassessment of ground structure.

[0038] In accordance with the eighth aspect of the invention, themeasurement of microseisms and the analysis of measured microseisms dataare continued until a relationship between phase velocity and wavelengthsuch that the ground structure can be determined with sufficientaccuracy is obtained so that the laminar structure and characteristicsof the ground can be determined expediently, rapidly and accurately on areal-time basis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a view showing the arrangement of oscillation sensors inan embodiment of the present invention;

[0040]FIG. 2 is a schematic view showing the measurement, analysis andassessment system of the same;

[0041]FIGS. 3 and 4 each is a flow chart for illustrating the procedureof said analysis and assessment;

[0042]FIG. 5 is a schematic view showing the measured and theoreticaldispersion curves;

[0043]FIG. 6 is a tectonic characteristic diagram obtained by inverseanalysis of the same according to the elastic wave propagation theory;and

[0044] FIGS. 7(a) through (d) are other arrangements of oscillationsensors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] The preferred embodiments of the present invention are describedin detail.

[0046] Referring to FIGS. 1 and 2 which illustrate a typical arrangementof oscillation sensors and a schematic representation of the systemembodying the principles of the invention, a three-component oscillationsensor 1 for detecting the vertical component of oscillation and thebidirectional horizontal component of oscillation as disposed in oneposition at ground level and a plurality of vertical-componentoscillation sensors 2 a, 2 b and 2 c as disposed at positionscorresponding to the apices B₁ through B₃ of a regular triangle with thelength of one side being R and the center of gravity coinciding withsaid one point A.

[0047] The above-mentioned oscillation sensors 1 and 2 a through 2 ceach comprises a speedometer and a displacement meter. In the abovearrangement, the short-period microseisms are simultaneously measured.

[0048] The detection signal outputs from the respective oscillationsensors 1 and 2 a through 2 c are amplified by a signal amplifier 3(FIG. 2) and then, fed to a data analyzer-assessor means 5, comprised ofa microcomputer, through an A/D converter 4 (interface).

[0049] The data analyzer/assessor means 5 analyzes the input dataaccording to the processing flow shown in FIG. 3 to compute provisionaldispersion curves and displays the curves on a cathode-ray tube CRT 6.

[0050] The measurement, analysis and assessment functions of the systemare described in detail below, referring to the flow charts shown inFIGS. 3 and 4.

[0051] Referring back to FIG. 1, the vertical and horizontal oscillationdetection signal outputs from the oscillation sensors 1 and 2 a through2 c located at points A and B₁ through B₃ are respectively amplified inthe signal amplifier 3 and, after A/D conversion, are fed to said dataanalyzer/assessor means 5 for data analysis ((1) in FIG. 3).

[0052] The oscillation data are filtered ((2), FIG. 3) and subjected tospectrum analysis inclusive of FFT (high-speed Fourier transformation)to determine the coherence, correlation coefficient and phase differenceof waves detected by the plural equi-spaced oscillation sensors for eachof the frequencies (f) involved (FIG. 3, (3)). When there is horizontalcomponent data (FIG. 3, (4)), the direction of approach of the dominantwave is determined (FIG. 3, (5)), the horizontal component data measuredby oscillation sensors 1 and 2 a through 2 c are analyzed in a directionperpendicular to the direction of approach of the waves (FIG. 3, (6)),and the movements of the ground surface in the planes parallel andperpendicular to the direction of approach of the waves are determinedas the locus of rotation of a particle on the ground surface (FIG. 3,(7)). Then, it is inquired if the predominant seismic wave is a surfacewave or not (FIG. 3, (8)). If the answer is affirmative, a provisionaldispersion curve (FIG. 3, (9)) is computed for the frequency relevant tothe surface wave.

[0053] The above series of measurement, analysis and assement isperformed with the length of sides of the tringle at the apices of whichsaid oscillation sensors 2 a, 2 b and 2 c are situated being varied from50 cm to 1 m to 2 m to 5 m and to 10 m or, alternatively, with aplurality of regular triangles varying in the length of sides being laidout and said oscillation sensors are disposed at the apices of eachtriangle.

[0054] Then, the provisional dispersion curves determined bymeasurements for regular triangles varying in length of their sides(FIG. 4, (1)-(2)) and if all the provisional dispersion curves areidentical, they are taken as the final dispersion curves (3). If theyare different, the fundamental mode and the higher-order mode arerespectively isolated and the degree of predominance is determined foreach frequency (4). Finally the final dispersion curve is determined.

[0055] With respect to the dispersion curve thus determined, inverseanalysis based on the theory of elastic wave propagation is carried outto estimate the laminar structure and characteristics of the ground.

[0056] In FIG. 5, the measured dispersion curve is represented by themark “o” and the ground structure determined by said inverse analysis isshown in FIG. 6. In FIG. 5, the solid line represents the theoreticaldispersion curve of the ground, which is well consistent with themeasured values, indicating that the characteristics of the ground canbe determined with reliable accuracy.

[0057] Now, the measurement of microseisms and the analysis of measuredmicroseisms are automatically continued until it can be judged that therelationship between phase velocity and wavelength which would beconducive to a sufficiently accurate structural determination. In thismanner, the laminar structure and characteristics of the ground can bedetermined expediently and accurately on a real-time basis.

[0058]FIG. 7(a) shows an example of arrangement whereoscillation'sensors 2 a through 2 e are disposed at apices of a regularpentagon, with an additional oscillation sensor 1 being disposed at thecenter of gravity of the pentagon.

[0059]FIG. 7(b) is an example of arrangement in which six oscillationsensors 2 a through 2 f are disposed at apices of a regular hexagon. Inthe arrangement illustrated in FIG. 7(c), oscillation sensors 1 and 2 athrough 2 d are disposed at apices of three equilateral triangles eachrotated through 45 degrees and having one of the apices in common. Inthe arrangement shown in FIG. 7(d), the circumference of a circle isequally divided into 12 sections and oscillation sensors 1 and 2 athrough 2 f are disposed on 6 of said circumferential sections and inthe center of gravity.

[0060] In the arrangement of FIG. 1 wherein short-period microseisms aresubjects of measurement, the length of one side is about 30 cm at theminimum and about 30 m at the maximum. In contrast, in the arrangementsshown in FIG. 7, the minimum length is about 20 cm and the maximum isabout 20 m, in terms of the radius of a circle, so that the scale ofmeasurement can be as much smaller.

What is claimed is:
 1. A method for estimating the laminar structure andother characteristics of a ground which comprises disposing anoscillation sensor adapted to detect the vertical component or thevertical and horizontal components of oscillations at each of at least 3points within a comparatively small area at ground level tosimultaneously measure microseisms at such plural points, if necessarychanging the locations of said points of measurement and repeating themeasurements, and analyzing the vertical oscillation data or verticaland horizontal oscillation data.
 2. A method for estimating the laminarstructure and other characteristics of a ground according to claim 1wherein said microseisms to be measured are short-period microseismsobserved at ground level.
 3. A method according to claim 1 wherein twooscillation sensors are disposed on each of 2 or more straight linespassing through the ground to be measured and dividing the azimuth of360 degrees into an even-number of equal sections or straight linesparalled thereto in such a manner that sensor-to-sensor distances areequal on said straight lines.
 4. A method according to claim 1 whereinsaid oscillation sensors are disposed at 3 or more points at qualspacings on the circumference of a circle and at one point in the centerof the circle, or at a total of at least 4 points.
 5. A method accordingto claim 1 wherein said oscillation sensors are disposed at apices of aregular triangle and the center of gravity thereof or a total of 4points.
 6. A method for estimating the laminar structure andcharacteristics of a ground according to any of claims 1 to 6 whichcomprises analyzing vertical oscillation data or vertical and horizontaloscillation data as measured simultaneously with said oscillationsensors, judging whether the oscillation waves detected by saidoscillation sensors are surface waves or not, and extracting andanalyzing the surface waves.
 7. A method for estimating the laminarstructure and characteristics of a ground according to any of claims 1to 6, which comprises analyzing the correlation coefficient and phasedifference of oscillation data measured by equi-spaced oscillationsensors for each frequency, performing the analysis repeatedly withsensor-to-sensor distances varied, analyzing and computing thefundamental mode of surface waves based on results of the respectiveanalyses and, if necessary, analyzing and computing the higher-modewavelengths.
 8. A method for estimating the laminar structure andcharacteristics of a ground according to any of claims 1 to 7 whichcomprises performing the measurement and analysis of microseismsrepeatedly until it can be concluded that a relation between phasevelocity and wavelength such as to give a sufficiently accurateassessment of ground structure is achieved.